In the present context, “trust” and “trusted” are used to mean that a device or service can be relied upon to work in an intended, described or expected manner, and has not been tampered with or subverted in order to run malicious applications. A specification for trusted computing has been developed by the Trusted Computing Platform Alliance and can be found at www.trustedpc.org.
A conventional trusted computing device comprises a tamper resistant tester which can test the device to ascertain if it is trustworthy. The outcome of the test can be used within the device or reported to another computing device attempting to communicate with it. An exemplary trusted component is described in the applicants co-pending International Patent Application Publication No. PCT/GB00/00528 entitled “Trusted Computing Platform”, the contents of which are incorporated by reference herein. If the outcome of the test is reported to another device, then that other device can use the report to determine a trust policy vis-a-vis the device offering the report, which controls its communication with the reporting device.
One disadvantage of a computing environment comprised of trusted computing devices of the kind mentioned above arises where a trusted computing device becomes compromised, e.g. by a virus. The trusted computing devices in the environment do not know if the other computing devices within the environment have been compromised unless they challenge the other computing devices to verify that they have not been compromised. The challenge-verification process can consume undesirable amounts of time and/or processing resources.